Method and apparatus for wet classification of solids



NOV. 15, 1960 o, EKSTRQM, JR 2,960,226

METHOD AND APPIRATUS FOR WET CLASSIFICATION OF SOLIDS Filed Nov. 1, 1957 2 Sheets-Sheet l XNVENTOR Glen 0. Ekstrem i /M @JM ATTORNEY Nov. 15, 1960 G. o. EKSTROM, JR 2,960,226

METHOD AND APPARATUS FOR WET CLASSIFICATION OF SOLIDS Filed Nov. 1, 1957 2 Sheets-Sheet 2 IqaI44 I I I I I m W H T h N W W m I M 6 g Y B United States Patent Oliver Incorporated, Stamford, Conn., a corporation of Delaware Filed Nov. 1, 1957, Ser. No. 693,898 8 Claims. or. zos rss This invention relates to the wet classification of pulps, containing a mixture of particle sizes ranging from coarse to fine to effect a controlled separation of the pulp solids into a coarse or oversize fraction containing solids of a size larger than the mesh of separation and a fine or undersize fraction containing solids smaller than such mesh of separation.

Classification, as heretofore practiced, has usually employed one of two general methods. The first of such methods is the so-called hydraulic classification in which pulp solids are kept in teeter or suspension by an uprising liquid stream which sweeps the fines out of the teeter bed as a relatively dilute suspension leaving coarse solids behind for subsequent discharge as a relatively dilute suspension. This method has found wide use, principally because of its ability to make extremely sharp cuts between fractions. However, this m'eth'o d is not all that is to be desired in all situations. This is so, because of its relatively high water requirements and the fact that the product fractions are relatively dilute, hence cannot be used in special operations which require a high solids concentration, such as closed circuit grinding, without further dewatering. Another disadvantage resides in the inability of hydraulic classification methods to handle feeds containing solids coarser than about 3 mesh sizes (Tyler screen) larger than the desired mesh of separation.

The other method in general prior use is the so-called mechanical classification in which mechanical elements, such as reciprocating rakes, are employed to agitate the pulp and move it through and out of a classification pool. Such agitation suspends the fines in the pool to be overfiowd therefrom while rakes remove the coarse fraction from submergence thereby producing a relatively concentrated fraction. Such prior method is noted for its ability to handle pulps containing a wide range of sizes and to produce fine and coarse fractions of relatively high solids concentration as compared with hydraulic classifiers and is particularly adaptable for closed circuit grinding operations in which the coarse fraction is recycled through a grinding mill until the desired fineness is attained. Another advantage of mechanical classification is its relatively low water requirement.

Despite the aforementioned advantages, mechanical classification suffers from certain disadvantages chief among which is its inability to make controlled sharp separations.

It is, therefore, a primary object of this invention to provide ways and means for the classification of pulps which ways and means combine the advantages of prior hydraulic classification and mechanical classification while eliminating or substantially minimizing the disadvantages of both suchprior methods.

Another object is the provision of apparatus for carrying but such classification, which apparatus is particularly adaptable to use in closed circuit grinding systems.

Still another object is the provision of apparatus enabling Patented Nov. 15, 1960 2. automatic control of the classification operation to yield a controlled sharp separation.

Briefly, the invention revolves about the concept of combining certain elements of hydraulic classification, to obtain its advantages of sharp separation, with certain elements of mechanical classification adapted to produce concentrated 7 fractions.

In somewhat more detail, the inveption envisions the introductionof pulp into a so-called fluidized zone in which particles are suspended by uprising hydraulic or operating liquid which, by control of the upward velocity of the water, effects sweeping fines of a size below the desired mesh of separation to overflow while coarser solids are allowed to settle into a subsided stratum maintained below the pointof water introduction. Thus, the coarse solids are allowed to settle as a dense pulp in a non;fli1idized area where mechanical elements move them to discharge. w

By the term fluidized" as used herein is meant that state wherein solids are maintained in suspension or teeter by action of the uprising liquid stream. The term non-fluidized refers to the condition wherein solids are not in teeter but rather are in a state of subsidence and generally supported by each other.

In accordance with the invention, the elevation or upper level of the mass of coarse or subsided fraction effects operation of a coarse fraction discharge controller by which the coarse fraction discharge rate is adjusted to maintain pre-determined operating conditions necessary to continuously yield the desired fractionation.

Since the invention obviates the necessity of maintaining the coarse fraction solids in a suspended or teeter condition, operating water requirements are significantly lessened as compared to conventional hydraulic classification. This is so because in accordance with the invention only the fine solids need be maintained in suspension.

In order that the invention may be more readily understood and carried into effect, it is further described, by way of example only, with reference to the accompanying drawings.

In the drawings:

Fig. 1 is a vertical sectional view of a preferred embodiment of the invention, certain elements being shown in elevation and others diagrammatically for purposes of clarity.

, Fig. 2 is a view taken in the plane of line 22 of Fig. l, certain elements being omitted for purposes of clarity.

Fig. 3 is a vertical sectional diagrammatic View illu'strating various operating zones in the classification tank.

The apparatus illustrated in Figs. 1 and 2 comprises a round tank 10 having a bottom 11 and a wall 12. The bottom comprises a shallow conical outer portion 13 and a central conical sump 14- provided with an outlet neck 15 having a controllable discharge valve 16, here illustrated as an inflatable annular valve structure 17 forming a co-nstrictable passage 18 and operable by air supplied through a conduit 19 which is valved as at 1%.

Air pressure variations for controlling underfiow discharge rates are elfected by operation of suitable automatic control devices responsive to density changes occurring within the tank, all as hereinafter described in more detail.

The tank has a conventional overflow launder 20 into which the fines fraction flows over a weir edge 20 a. A suitable structure 21 is provided to support a rotary rake structure 22. Such rake structure includes a vertical hol low shaft 23 supported by bearing means 24 and is .provided with a set of evolute shaped raking elements; or blades 25 extending from the lower end of the shaft adja- 'cent the tank bottom. Secured to the bottom of the rake structure is a scraper or stirring member 18 shaped to the contour of the central sump to facilitate passage of material from the sump into the discharge neck. A suitable drive mechanism for rotating the rake structure is indicated collectively at 29 and comprises a drive my tor 30, power transmitting means 31 and a reduction gear 32 in engagement with the vertical shaft 23.

Pulp to be classified is supplied through a feed pipe 33 leading to a cylindrical feedwell 34 whence it falls onto a shallow feed distributing tray 35. The pulp flows from tray 35 over a weir edge 36 into and through an annular passage 37 between the feedwell and the tray.

Hydraulic operating water is introduced by an induction pipe system which uniformly disperses water in a horizontal plane spaced above the tank bottom. The illustrated induction pipe system comprises a plurality of concentric conduits or manifolds 38, each having a plurality of downwardly directed jet openings 39. Short vertical supply pipes 38a connect the annular pipes to radially extending headers 40 and 41 respectively connected to supply pipes 42 and 43 emanating from a main supply conduit 44, valved as at 45.

As part of the automatic control system there is provided an open-ended hydrostatic tube or probe 46 having its lower end positioned a distance d into the tank and terminating a distance 03 above the normal level L, of subsided or non-fluidized coarse solids. Changes in the elevation of water in pipe 46, due to changes in the elevation L; of subsided solids, effect an increase or decrease, as required, in the discharge opening 18 which in turn effects restoration of the subsided solids level and the water level in the probe to their proper elevation. Fresh clear water may be added to the tube by any suitable valve such as 46a.

The control is accomplished by means of the illustrated control system. Such system, which is described in detail in U.S. Patent 2,715,463, comprises a conduit 47 connecting the probe 46, and a conduit 48 connecting the discharge valve 16. The system is regulated by a master control unit 49. High pressure operating air is admitted through a supply conduit 50 whence it passes through a filter 51, a T restriction device 52', a reducing valve 53 and a booster relay device 54.

Referring to Fig. 3, it will be seen that there exists within the classifier tank several zones of varying conditions and degrees of solids-concentration density. Such zones include a general subsided zone Z which includes a lower stratum Z in which solids are being raked to discharge and an upper zone Z which may be termed a variable density zone which preferably extends above the plane of water introduction, hence serving as distributing 'means for such water.

Above zone Z there extends a fluidized or teeter zone 2.; in which hydraulic classification occurs and in which the density of the suspension decreases from top to bottom due to the presence of coarse solids in the lower portion settling toward the non-fluidized zone. The overall zone Z may be envisioned as including an intermediate zone Z which functions as a transitional zone with the prevailing conditions therein being of coarse solids descending in a hindered settling or semi-teeter state; and a superjacent zone Z which may be characterized as a state of true teeter in which final separation of undersize or fine solids from relatively small coarse particles occurs, the undersize or fines eventually overflowing the weir and the coarse solids eventually settling into the subjacent non-fluidized zone.

In the true teeter zone 2 the particles, both undersize and oversize, may be considered to be in a state of true teeter in which support is derived solely from the-rising water rather than by contact with other solids.

It is important to note that the upper level L of the non-fluidized zone Z will be maintained at a minimum operating elevation substantially at or slightly above the elevation of introduction of hydraulic water. If such but level rises above such preset elevation it will cause a corresponding rise of all zones within the tank and this will bring a denser suspension into proximity with the lower end of the probe 46 thus increasing the elevation or super-elevation of water within such probe. This actuates the control mechanism and causes the discharge valve to open, thus increasing the coarse solids discharge rate until the water level in 46, and the upper level of subsided solids have again reached their proper elevations. Conversely, should the upper level L of subsided zone Z fall from a preset level, the control will respond to restrict the discharge passage and raise such level. In this manner, by compensating for density variations at a selected elevation in the tank, a pro-determind reference level of the non-fluidized zone is automatically maintainable. By maintaining such level constant, conditions in the true classification zone Z also remain constant, hence operation of the classifier, so far as the fractionation cut is concerned, will also be maintained substantially constant.

In connection with the reference level L of the nonfiuidized zone, it is to be noted that it should be maintained substantially level, and this is insured by the stirring action of the mechanical elements in the nonfluidized zone. It is also to be noted that the mechanical elements should be such as to enable movement of the settled solids toward the discharge point at varying rates to'accomrnodate changes in discharge rate. This can be accomplished, but a convenient method is to pro vide rakes of a solids moving capacity greater than the normal or designed discharge rate so that when such discharge rate increases, the blades, due to decreased resistance at the discharge point, will automatically move solids thereto at an increased rate to accommodate such increased discharge rate.

It is to be understood that the invention itself includes arrangements in which the tank as well as the solids conveying means may be of a construction other than illustrated. For instance, the water induction pipe system may take various other shapes, and it may be rotatable with the rotary structure instead of being stationary.

I claim:

1. Combined mechanical thickening and hydraulic classification apparatus comprising a tank having a pcripheral overflow launder at the top thereof and a conical shaped bottom, rotary rake means adjacent said bottom and a centrally located outlet to remove a coarse fraction from said bottom, feed well means to introduce solids to be classified into said tank, an induction pipe system to uniformly disperse operating water in a horizontal plane spaced above said rake means and adapted to allow a coarse fraction of said solids to pass to said bottom through said horizontal plane from a teeter bed above said plane, means to conduct operating water under pressure to said induction pipe system, and density-responsive means responsive to density changes in the teeter bed for controlling the withdrawal of said coarse fraction from said bottom outlet for maintaining a zone of thick ened coarse material relative to said horizontal plane.

2. Apparatus according to claim 1 wherein said induction pipe system comprises a plurality of concentric conduits having a plurality of openings to eject jets of operating water into the space between said horizontal plane and said rake means, and said concentric conduits being radially spaced relative to each other to allow said coarse solid fractions to pass therebetween.

3. The method of continuous classification treatment of pulp containing a range of mixed solids from fine to coarse by hydraulic fractionation in a teeter bed of controllable density for producing an overflow fraction and an underflow fraction, which comprises; subjecting the solids to a rising stream of operating water in a fluidizing zone providing teeter bed conditions causing fines to reach the overflow while allowing coarse particles to sink, accumulating and thickening the coarse particles in a nonfluidized storage zone which is subjacent to the fluidized teeter zone and provides a variable reference level for controlling the fractionation in the fluidized zone, introducing operating water dispersingly in a horizontal plane at the top of said non-fluidized zone, continuously withdrawing thickened coarse fraction material from said non-fluidized zone, and correctively controlling the rate of coarse fraction withdrawal responsive to density variat-ions in the fluidized zone and in a manner to maintain said reference level of non-fluidized material at least as high as said horizontal plane of water introduction.

4. The method according to claim 3, which comprises maintaining said storage zone sufficiently stirred to insure substantial horizontal evenness of said reference level of the non-fluidized zone.

5. The method according to claim 3, which comprises maintaining said reference level of the non-fluidized zone above said horizontal plane of water introduction to provide for a substantially non-fluidized water dispersing transitional zone of coarse solids between said fluidized Zone and said plane of water introduction.

'6. The method according to claim 3, which comprises mechanically engaging and moving the material in the bottom strata of the non-fluidized zone to a discharge zone at a rate greater than the rate of underflow discharge.

7. The method according to claim 3, which comprises 6. mechanically engaging and moving the material in the bottom strata of the non-fluidized zone to a discharge zone at a rate greater than the rate of underflow discharge, while maintaining said reference level of the non-fluidized zone above said horizontal plane of water introduction to provide for a substantially non-fluidized water-dispersing transitional zone of coarse solids between said fluidized zone and said plane of water introduction.

8. The method according to claim 3, which comprises maintaining said storage zone sufliciently stirred to insure substantial horizontal evenness of said reference level of the non-fluidized zone, While maintaining said reference level of the non-fluidized zone above said horizontal plane of water introduction to provide for a substantially nonfluidized water-dispersing transitional zone of coarse solids between said fluidized zone and said plane of water introduction.

References Cited in the file of this patent UNITED STATES PATENTS 801,200 Bailey Oct. 10, 1905 1,511,643 Trent Oct. 14, 1924 2,105,101 Stelley Jan. 11, 1938 2,560,809 Martin July 17, 1951 2,715,463 Fitch Aug. 16, 1955 2,715,886 Martin Oct. 16, 1956 

